The invention relates to high temperature wafer boats suitable for carrying a plurality of semiconductor wafers during high temperature processing thereof in diffusion tubes, and more particularly to improvements in wafer boats to avoid stressing or binding of wafers as a result of thermal expansion thereof, and still more particularly to improvements that avoid generation of silicon dust and quartz dust or the like due to "rattling" or movement between supported wafers and the wafer-supporting slots of the wafer boats.
It is well known that numerous cleaning, coating and diffusion operations are required in the fabrication of semiconductor wafers, and that often fifty-wafer quartz boats are used, wherein there is a three/thirty-seconds (3/32) of an inch center-to-center spacing between the wafers for diffusion operations, whereas for most other cleaning operations, plastic carriers are used. The plastic wafer carriers also have long vertical side grooves or wafer guides, the bottoms of which are approximately one hundred mils wide and the sides of which are tapered outwardly by approximately two degrees. The most efficient, highest yield semiconductor manufacturing facilities include expensive equipment and require extensive precautions on the part of workers to maintain the wafer fabrication environment at the highest possible level of purity. For example, expensive laminar air flow systems, and expensive dust filters often are used. The most modern facilities completely eliminate use of tweezers and manual handling of wafers in order to reduce the amount of silicon dust that is produced, and all workers wear clean gowns, hair nets, and in some cases, even face masks. However, due to build-up of static electricity on various wafer handling components, even small amounts of silicon dust can become attached to wafer surfaces or to surfaces that touch wafers. For example, silicon dust can become attached to the sides of the grooves of the above-mentioned plastic wafer carriers, and when the wafers are transferred into such carriers, the periperal portion of the active face of the wafer may slide against such silicon dust, causing defects in one or quite a few peripheral circuits on the wafer. This, of course, reduces yield and increases the overall cost per unit of the circuits being manufactured.
The prior quartz diffusion boats cause generation of silicon dust and quartz dust because the wafer-supporting grooves thereof generally have shapes and tolerances which allow the wafers to "rattle" or move in these grooves as the boat is moved during various operations. This movement or rattling results in abrasion of edges of the wafers against the bottoms of the wafer-supporting grooves, thereby causing the breaking off of minute particles of silicon and quartz, which then may adhere to surfaces of semiconductor wafers. Such abrasion also occurs for other types of wafers, such as gallium arsenide, and for other types of carriers, such as polycrystalline silicon or silicon carbide. The particulates produced by abrasion therebetween are also highly undesirable and are known to cause defects in integrated circuits.
Most prior quartz wafer boats fall into one of three categories, namely, "four member" boats, "three member" boats, and "shell" boats. All of these prior wafer boats have the problem that wafers held therein are subject to a considerable amount of "rattling" or rolling from side to side during normal "handling" of the boats, either by humans or by automated machinery, during wafer manufacturing operations. The wafer movement or "rattling" in such boats occurs because of tolerances that need to be provided in each of the slots in which the peripheral edges of the wafers rest, and produces minute particles of the wafer or the boat, for example, silicon dust and quartz dust, if the wafers are silicon and the boat is made of quartz, of course. Another problem that characterizes the above high temperature wafer boats is that when a boat load of wafers is placed in the hot zone of a furnace, its temperature is typically raised to temperatures in the range of 600.degree. to 1250.degree. Centigrade or greater. Silicon has a much greater coefficient of thermal expansion than any of the common materials of which wafer boats are made, namely quartz, polycrystalline silicon, and silicon carbide. The greater thermal expansion of the wafers can cause "binding" of the wafers as they expand (more with respect to temperature than the wafer boats), unless sufficient clearance is provided between the wafer edges and the bottoms of the slots in the upper members or slot-containing members. Such stresses can cause cracking of wafers, and can also cause generation of defects in the crystal lattice structure, resulting in reduced integrated circuit yields, and hence in decreased profitability.
The known "four member" boats include two bottom members that support the weight of the wafers. The bottom members are positioned symmetrically about the center of gravity of the wafers supported thereby. The two side members or upper members also are symmetrically positioned, higher up and further outward relative to the bottom members. Usually, sufficient clearance is allowed between the bottoms of the grooves in the upper members and the edges of the wafers to allow for wafer expansion. As a result, the wafers can rock back and forth between the bottoms of the slots of the side members, when the boat is moved or subjected to vibration, for example by robotic machines, generating minute particles of the wafer material and the boat material, such as quartz dust and silicon dust. The bottom members can be moved further apart to reduce the tendency of the wafers to rock to and fro in this manner, but this results in the wafers being held less perfectly vertical and parallel, which in turn leads to a reduction in the uniformity of flow of reactant gases between wafers, and a consequent reduction in yield of the individual integrated circuits being formed in the wafers. If the bottom members are positioned close together, the wafers are more unstable and more apt to rotate about their vertical axes when the boat is subjected to movement or vibration, since the areas of contact are closer together. This kind of rotation or "rattling" movement between the wafers and the carrier also causes generation of silicon and/or quartz particulates or the like. Such rotation also decreases the preciseness of the parallel relationship between wafers and adversely effects the uniformity of diffusion and/or deposition processes that form integrated circuits in the wafers. Furthermore, if the two bottom members are moved further apart to provide more stability of the supported wafers, the forces produced by binding as the wafers expand at high temperatures are much greater than if the bottom members are close together, leading to buckling, possible wafer cracking, and likely generation of defects in the silicon lattice structure at high temperatures.
The above-mentioned prior art "three member" wafer carriers have a single bottom member located directly below the center of gravity of the wafers, and two symmetrically positioned side members. Enough clearance is allowed in the bottoms of the slots of the side members in relation to the wafer diameter to accommodate wafer expansion at high temperature. In such three member wafer boats, the wafers teeter back and forth on the center member and also to rest on the "bottom" of one side member or the other. The wafers in "three member" wafer boats have an even greater tendency to roll back and forth or rattle during handling of the wafer boat than the above-described "four member" wafer carriers; however, they tend to hold the wafers more precisely vertical and parallel due to the greater distance between the bottom member and the upper side support members. In the shell-type carriers, in which a solid semicylindrical section of quartz has semicircular slots, the radius of curvature of the semicircular parallel slots must be slightly greater than the radius of the semiconductor wafers to avoid severe binding at high temperatures. Hence, the wafers rock back and forth in the slots, again generating quartz dust and silicon dust.
In a fourth type of wafer carrier, described in commonly assigned co-pending application "Semiconductor Wafer Diffusion Boat and Method", Ser. No. 544,588, filed Oct. 24, 1983, by Robert M. Butler and incorporated herein by reference, wafer grooves in adjacent members or rails are provided with oppositely sloped surfaces and opposed vertical surfaces, respectively, that maintain each wafer very precisely vertical, so that large wafers are held more precisely parallel than in any prior carrier. The two members contain the oppositely sloped wafer grooves that are positioned relatively close together to support the bottom edge of each wafer and to prevent excessive stress due to thermal expansion of the wafers.
Those skilled in the art know that numerous efforts are made to effectuate "gentle" handling of wafer boats to avoid the above-described wafer rattling. Considerable efforts are made in the field of automated wafer handling to subject wafer boats to minimum amounts of acceleration, deceleration, and vibration but nevertheless, prior wafer boats, except for the one described in co-pending application Ser. No. 544,588 mentioned above, are subject to considerable rattling within the boat or carrier, despite such precautions and efforts.
There clearly is a continuing need for further improvements in the field of providing wafer carriers that carry a plurality of semiconductor wafers in perfectly parallel, equally spaced relationship to each other, and yet avoid the above-mentioned difficulties, especially the difficulties that lead to wafer rattling and consequent generation of silicon dust, and/or quartz dust and the like and which also avoid stresses within the wafers, especially during high temperature processing operations.